1. Field of the Invention
This invention relates to a pulsed eddy current proximity detector and process for locating sub-surface objects such as pipelines and cables. The pulsed eddy current proximity apparatus and process of this invention may detect the presence, assess the range, and assess the size and direction of an underground pipeline or cable with equipment which may be wholly mounted on earth removal or digging apparatus. The pulsed eddy current proximity detector may have its transmit/receive coil directly emplaced in or mounted on an excavator implement such as a backhoe bucket to transmit and receive electromagnetic radiation to detect conducting objects prior to contact of the excavator implement with such hidden objects. The apparatus and process very reliably makes such detections and may automatically shut down the excavator implement prior to contact with the underground metal object.
2. Description of the Prior Art
A wide number of technologies have been considered for detection of sub-surface or hidden objects. These technologies include nuclear, acoustic, gravitational, magnetic and electromagnetic, such as infrared, microwave and low-frequency magnetic. Both active and passive detection systems have been attempted. Nuclear systems have the inherent safety disadvantages. The desirability of mounting the entire proximity detector on the excavator itself eliminates radar/acoustical, shortwave-longwave or electromagnetic induction active systems. Sonic systems have the disadvantage of being dependent upon good soil contact.
A wide number of metal detectors have been used as buried treasure locators, in geophysical exploration, in law enforcement, and in enhancing airport security. These types of detection devices generally operate to create a near field of continuous wave electromagnetic forces about a central inductive coil. When a metallic object is brought within the field, an impedance change occurs, resulting in the objects detection. Such detectors are sensitive to variations in the soil giving rise to false signals which cannot be tolerated. Further, such detectors do not provide desired discrimination between the desired object, such as a pipe, and debris, such as a metal can. Many continuous electromagnetic wave type detectors are typically constructed having a transmit coil mutually coupled with a receive coil. The requirement of mutually coupled coils inhibits the use and application of these type devices. Because the geometry of the coils are critical for operation, the device must be constructed of rigid members to maintain precise relative coil placement. These type devices are very sensitive to slight jarring or impact which may cause coil movement rendering the device unsuitable for heavy duty applications such as in earth removal operations. Attempts have been made to improve coil arrangements to eliminate the requirement for mutual coupling as taught in U.S. Pat. No. 3,588,687. A more comprehensive review of potential technologies for underground pipeline detectors is set forth in Backhoe Pipeline Damage Prevention by MIDDARS, Jack E. Bridges, Workshop for Gas Distribution and Safety Instrumentation, Kissimmee, Fla., Feb. 1-3, 1983, directed by Institute of Gas Technology for Gas Research Institute and is incorporated herein in its entirety.
More recent efforts to improve upon metal detector sensitivity have been directed to the use of pulsed eddy current detectors in which a pulsed magnetic field is directed toward a target and induces eddy currents in conductive targets. Voltages induced by the decay of the eddy currents are detected. Mutual coupling is not required between the transmit and receive coils. See "Pulse Induction Metal Detector", J. A. Corbyn, Wireless World, Vol. 86, No. 1531 (March 1980) and No. 1532 (April 1980). These articles teach the use of circuitry to eliminate background clutter and noise attributed to the magnetic viscosity properties of earth media and are incorporated herein in their entireties.